The present invention relates to the general field of mixers for mixing concentric gas streams from a bypass turbomachine. The invention relates more particularly to a mixer of the daisy type for a converging-stream nozzle.
Sound pollution is nowadays one of the major concerns of engine manufacturers who are receiving more and more objections to the sound nuisance of their turbomachines. The sources of noise in a turbomachine are numerous, but that it has been found that the noise of the jet leaving the nozzle is the predominant noise during airplane takeoff. Certification authorities are becoming more and more strict in terms of noise emission from turbomachines, and manufacturers have been requested to make efforts to reduce the noise that turbomachines generate, and in particular the jet noise at the outlet from the nozzle.
Typically, a converging-stream nozzle of a turbomachine comprises a primary cover centered on the longitudinal axis of the turbomachine, a secondary cover disposed concentrically around the primary cover so as to define a first annular channel along which an outer stream (or cool stream) flows, and a central body disposed concentrically inside the primary cover so as to define a second annular channel along which an inner stream (or hot stream) flows, with the secondary cover extending beyond the primary cover.
Generally, a converging-stream nozzle also includes a mixer mounted at the downstream end of the primary cover. Such a mixer is designed to reduce jet noise at the outlet from the nozzle by forcing mixing to take place between the cool stream and the hot stream prior to their ejection. It is well known that such reductions in noise are obtained by increasing the degree of mixing between the cool stream and the hot stream coming from the turbomachine.
Amongst mixers for converging-stream nozzles, one particular known type of mixer is of the daisy type that is in the form of a substantially sinusoidal portion defining inner lobes and outer lobes distributed around the circumference of the primary cover of the nozzle. Reference can be made for example to U.S. Pat. Nos. 4,077,206 and 4,117,671.
With such a daisy type mixer, the inner lobes form gutters guiding the cool stream radially towards the second channel in which the hot stream flows, and the outer lobes form other gutters guiding the hot stream radially towards the first channel in which the cool stream flows. Thus, at the outlet from the mixer, the cool and hot streams mix in shear in a direction that is substantially radial. This mixing enables turbulence to be generated presenting an axis of rotation that is generally axial and of a magnitude that depends mainly on the stream ejection conditions (turbomachine bypass ratio, shear between the cool and hot streams) and on the conditions with which the bottoms of the lobes of the mixer are fed.
Unfortunately, when the ejection conditions of the streams and the feeding of the bottoms of the lobes are not optimized, the magnitude of the turbulence generated by a daisy type mixer is not sufficient for obtaining really effective mixing between the cool stream and the hot stream, thus limiting the extent to which the jet noise levels can be reduced during airplane takeoff.